Waxy products of a FT hydrocarbon synthesis process, particularly the products of a cobalt and/or iron based catalytic process, contain a high proportion of normal paraffins. Primary FT products provide notoriously poor cold flow properties, making such products difficult to use where cold flow properties are vital, e.g. diesel fuels, lube oil bases and jet fuel. It is known in the art that cold flow properties of a middle distillate, such as jet fuel, can be improved by increasing the branching of the paraffins of distillates within the proper boiling range, as well as by hydrocracking and hydroisomerising heavier components. Hydrocracking, however, produces smaller amounts of gases and light products, which reduce the yield of valuable distillates. There remains an incentive for a process to maximize middle distillates obtained from FT waxes having good cold flow properties and a high Cetane number.
The middle distillate fuel described in this invention is produced from a highly paraffinic synthetic crude (syncrude) obtained from synthesis gas (syngas) through a reaction like the FT reaction. The FT primary products cover a broad range of hydrocarbons from methane to species with molecular masses above 1400; including mainly paraffinic hydrocarbons and much smaller quantities of other species such as olefins, and oxygenates.
The prior art teaches in U.S. Pat. No. 5,378,348 that by hydrotreating and isomerizing the products from a Fisher-Tropsch reactor one can obtain a jet fuel with freezing point of −34° C. or lower due to the isoparaffinic nature of this fuel. This increased product branching relative to the waxy paraffin feed corresponds with a Cetane rating (combustion) value less than that for normal (linear) paraffins, depicting that an increase in branching reduces the Cetane value of paraffinic hydrocarbon fuels.
Further, WO 97/14769 discloses diesel fuels having excellent lubricity, oxidative stability and high cetane number produced from the non-shifting Fischer-Tropsch processes and having >95 wt % paraffins with an iso to normal ratio of from 0.3 to 3.0. No mention is made regarding the effect of branching on the cold flow properties or the cetane number.
Still further, WO 98/34998 discloses a process for producing additve compositions, especially via a Fischer-Tropsch reaction, useful for improving the cetane number or lubricity of a middle distillate diesel fuel. The additive is prepared by fractionating the products of a Fischer-Tropsch reaction into a low boiling 371 deg C. fraction and a high boiling fraction, and hydroisomerising the high boiling fraction into a low boiling fraction and blending the low boiling fraction and the hydroisomerised high boiling fraction to produce the additive having >90 wt % C16 to C20 paraffins of which >50 wt % are isoparaffins. This disclosure does not disclose that a diesel fuel having good cold flow properties and high cetane number can be produced, only an additive, also the disclosure requires hydroisomerisation of a high boiling fraction which leads to a loss of material from the diesel boiling range into lighter material and to the formation of branched isomers, which leads to Cetane ratings less than the corresponding n-paraffins. The disclosure also does not address the issue of cold flow properties simultaneously with high a Cetane number.
Surprisingly, it has now been found by the applicant, that a hydroprocessed middle distillate, such as diesel, may be produced having a high Cetane number as well as good cold flow properties. The middle distillates of the present invention could be used on their own or in blends to improve the quality of other diesel fuels not meeting the current and/or proposed, more stringent fuel quality specifications.